Improve the curve tessellation subdivision of the path renderers.

The stencil-and-cover and tessellating path renderers currently
tessellate curves to linear segments to within one screen space pixel.
This is fairly poor accuracy, as can be seen from the strokecircle GM:
there are noticeable rotating bumps on the circles.

Improving the tolerance to 0.25 pixel eliminates the bumps, and
approximates the 16x supersampling of the raster path.
This does have a performance hit: 3-6% on desktop on
the IE chalkboard demo, ~1% on Nexus 7 2013.

Note: this will require rebaselining a number of GPU and  MSAA images
in Skia, but nothing in Chrome (yet).

BUG=skia:3731

Review URL: https://codereview.chromium.org/1072273007

3 files changed, 10 insertions, 2 deletions

diff --git a/src/gpu/GrDefaultPathRenderer.cpp b/src/gpu/GrDefaultPathRenderer.cpp
index 531b5e5f4a..65daa7ca89 100644
--- a/src/gpu/GrDefaultPathRenderer.cpp
+++ b/src/gpu/GrDefaultPathRenderer.cpp
@@ -658,7 +658,7 @@ bool GrDefaultPathRenderer::internalDrawPath(GrDrawTarget* target,
         }
     }
 
-    SkScalar tol = SK_Scalar1;
+    SkScalar tol = GrPathUtils::kDefaultTolerance;
     SkScalar srcSpaceTol = GrPathUtils::scaleToleranceToSrc(tol, viewMatrix, path.getBounds());
 
     SkRect devBounds;
diff --git a/src/gpu/GrPathUtils.h b/src/gpu/GrPathUtils.h
index c15fd7ae10..250b0a2324 100644
--- a/src/gpu/GrPathUtils.h
+++ b/src/gpu/GrPathUtils.h
@@ -168,5 +168,12 @@ namespace GrPathUtils {
     // If you transform the points the lines will also need to be transformed. This can be done
     // by mapping the lines with the inverse-transpose of the matrix used to map the points.
     void getCubicKLM(const SkPoint p[4], SkScalar klm[9]);
+
+    // When tessellating curved paths into linear segments, this defines the maximum distance
+    // in screen space which a segment may deviate from the mathmatically correct value.
+    // Above this value, the segment will be subdivided.
+    // This value was chosen to approximate the supersampling accuracy of the raster path (16
+    // samples, or one quarter pixel).
+    static const SkScalar kDefaultTolerance = SkDoubleToScalar(0.25);
 };
 #endif
diff --git a/src/gpu/GrTessellatingPathRenderer.cpp b/src/gpu/GrTessellatingPathRenderer.cpp
index c80b693ef5..1c1663dbf3 100644
--- a/src/gpu/GrTessellatingPathRenderer.cpp
+++ b/src/gpu/GrTessellatingPathRenderer.cpp
@@ -1392,7 +1392,8 @@ public:
             c.sweep_lt = sweep_lt_vert;
             c.sweep_gt = sweep_gt_vert;
         }
-        SkScalar tol = GrPathUtils::scaleToleranceToSrc(SK_Scalar1, fViewMatrix, pathBounds);
+        SkScalar screenSpaceTol = GrPathUtils::kDefaultTolerance;
+        SkScalar tol = GrPathUtils::scaleToleranceToSrc(screenSpaceTol, fViewMatrix, pathBounds);
         int contourCnt;
         int maxPts = GrPathUtils::worstCasePointCount(fPath, &contourCnt, tol);
         if (maxPts <= 0) {